In general, electrophotographic or electrostatic-recording full-color image forming apparatuses form images using four colors: yellow, magenta, cyan, and black, and mainly following two methods are known.
One is an image formation apparatus of the four-cycle method which is provided with one photosensitive member and a plurality of developing units. In this method, electrostatic latent images are sequentially formed on one photosensitive member in accordance with image information. These electrostatic latent images are developed using toner images of a plurality of colors, and the toner images of the respective colors are sequentially transferred onto an intermediate transfer belt, from which the toner images are re-transferred onto a recording sheet, or directly onto recording paper in such a manner that the toner images are superimposed on one another. Thus, a color image is formed.
The other is an image formation apparatus of the tandem method which is provided with one photosensitive member and one developing unit per color. In this method, electrostatic latent images are formed on respective photosensitive members in an image formation apparatus in accordance with image information. These electrostatic latent images are developed using toner images corresponding to the respective colors, and these toner images are sequentially transferred onto an intermediate transfer belt, from which the toner images are re-transferred onto recording paper, or directly onto recording paper so that the toner images are superimposed on one another. Thus, a color image is formed.
In the above image forming apparatuses, in order to control the density of an image to be formed, image forming conditions for forming an electrostatic latent image on a photosensitive member, such as an amount of exposed light, a developing bias, and a charging potential, are controlled. However, even if these image forming conditions are the same, the densities of images to be formed change due to influences such as changes in various quantities of state of an image forming apparatus with time, including the amount of charge of toner, the sensitivity of a photosensitive member, and transfer efficiency, and changes in environmental conditions such as temperature and humidity.
Conventionally, therefore, the density of a toner image transferred onto a photosensitive member or an intermediate transfer belt is detected, and image forming conditions such as a charging potential, an amount of exposed light, and a developing bias are feedback-controlled on the basis of the detection result.
For example, there is one in which a patch image is irradiated with light and the density of the patch image is detected based on the amount of light reflected from the patch image (the amount of reflected light) (see, for example, PTL 1).
There is another in which a density-measuring toner image borne on a photosensitive member or an intermediate transfer belt is irradiated with light and the height of the toner image is measured based on a light receiving position on a line sensor that receives reflected light from the toner image. Here, the higher the density of the toner image is, the larger the amount of toner (the amount of adhering toner) used to form the toner image is and therefore the greater the height of the toner image is. Further, the lower the density is, the smaller the amount of toner (the amount of adhering toner) used to form the toner image is and therefore the lower the height of the toner image is. Thus, the height of a toner image measured based on a light receiving position on a line sensor is converted into density as an amount of adhering toner (see, for example, PTL 2).